1. Technical Field
The present invention relates to technology for accessing a semiconductor memory according to instructions of an accessing apparatus, in particular to technology for performing the access in accordance with the amount of current that can be applied by the accessing apparatus.
2. Background Art
Since being compact, light and slim and allowing easy manipulation, SD cards and other memory cards equipped with flash memories therein are widely employed in digital apparatuses such as digital still cameras. The storage capacities of commercially available memory cards are in general from 16 to 512 MB, and the recording rates are up to 10 MB/s or so.
For recording data with large storage space requirements, such as video and audio data in DV format, however, the above storage capacities and recording rates are not sufficient. In the case of data with a reproduction rate of 4 MB/s, for example, only two-minute's worth of data can be recorded, and the writing process takes about one minute.
In order to solve these problems, Patent References 1 and 2 disclose memory card drives capable of mounting multiple memory cards thereon. According to the disclosed technology, a large storage capacity is attained by using more than one memory card placed in a memory card drive. In addition, an improvement in the recording rate is achieved by writing data into multiple memory cards in parallel using striping, which is known as one of the RAID formats.
In the case of using such a memory card drive connected to a personal computer (hereinafter, “PC”) or another electric apparatus, the amount of the current that the electric apparatus is able to supply to the memory card drive differs from one electric apparatus to another. When it comes to PCs, for example, the current that can be supplied by a PC to a memory card drive is defined in the PC card standard as 1 A. However, the available supply current practically varies among types of PCs due to their heat release characteristics and the like.
When two or more memory cards are operated in parallel using a memory card drive, the consumption current of the memory card drive increases in proportion to the number of memory cards in operation. Operating four memory cards each having a consumption current of 200 mA, for instance, claims a total consumption current of 800 mA. Accordingly, PCs applicable to this memory card drive are limited only to those feasible to supply 800 mA or more current to the memory card drive.
<Patent Reference 1> Japanese Laid-Open Patent Application Publication No. 2002-189992
<Patent Reference 2> Japanese Laid-Open Patent Application Publication No. 2000-207137